This invention relates to high power (nonlinear) testing of microwave transistors (DUT). When the transistors are driven in their nonlinear operation regime their internal output impedance is very low. An impedance tuner used to match the transistor must also conjugate-match this impedance, i.e. the reflection factor presented by the tuner to the DUT must have the same amplitude and the opposite phase: Γtuner=Γ*DUT. Passive impedance tuners can reach maximum reflection factors |Γtuner| of the order of 0.95, corresponding (in a 50Ω system) to impedances of 2.4Ω. The insertion loss, created by RF cables, test fixtures etc. between the DUT and the tuner reduces the available tuning range at the DUT reference plane and thus the capacity of the passive tuner to match the transistor. The only unconditional remedy to this limitation is using active systems, i.e. test systems whereby a signal, coherent with the signal which is injected into the input and exits from output of the DUT, is reverse-injected simultaneously into the output of the DUT, coming from the load, and creates a “virtual” load. This additional signal can be the only one injected, in which case we speak of purely “active” load pull, or it can be superimposed to signal reflected by a passive tuner, in which case we speak of “hybrid” load pull; obviously, if only a (mechanical or electronic) tuner is present, we speak of “passive” load pull. In both active injection cases (pure active and hybrid) the objective is to reach and conjugate match the internal impedance of the transistor; in general terms a standard requirement is a dynamic tuning range reaching a reflection factor |Γ|=1 at the DUT reference plane (corresponding to an internal DUT impedance close to 0Ω); because of the above mentioned insertion losses between DUT and tuner, however, it is necessary that, at the tuner reference plane the generated reflection factor Γtuner be |Γtuner|>1. The objective of this invention is a hybrid (active plus passive) tuner apparatus, combining a forward signal injection mechanism with a passive electronic tuner, allowing |Γ|≥1. It must be clarified at this point that “electronic” does not mean “active”. Electronic tuners, as disclosed here, are passive, but not electro-mechanical.
Passive automatic (remotely controlled) tuners are either electromechanical (see ref. 5) or electronic (see ref. 6). Electromechanical tuners cover high frequency bandwidth (are wideband), generate high reflection factor (Gamma), are linear, have high tuning resolution, but they are slow, because of the mechanical movement. Electronic tuners use PIN diodes (see ref. 7), and have smaller bandwidth, lower maximum reflection factor Gamma, lower linearity and resolution than mechanical tuners, but they are extremely fast (they switch states in milli-seconds versus seconds of mechanical tuners); so in essence we are talking about an increased speed ratio (or reduced tuning time) of 1000:1. For a number of applications electronic tuners, if enhanced with active modules, as in this invention, can reach maximum reflection factor |Gamma|≥1 and can exploit their high tuning speed. And as modern test technologies evolve into automatic testing a large number of on-wafer chips, speed is of essence and may overcome other, above mentioned, comparative weaknesses of electronic tuners.